Pulmonary fibrosis is a lethal disease for which no successful therapy is available. Experimental silicosis is an excellent model to study this disease. In the US, more than 2 million people are exposed to silica every year. Macrophages play a fundamental role in lung fibrosis. Stem cell-based therapy for fibrotic lung diseases has yet to be realized. We previously reported that systemic administration of bone marrow derived mesenchymal stem cells (MSCs) ameliorates experimental lung fibrosis. The number of MSCs retained in the injured lung is small and inconsistent with the postulate that MSCs promote lung homeostasis by their capacity for tissue regeneration. MSCs demonstrate an important paracrine activity through a secretome that influences the biology of target cells and we previously reported that beneficial effects of MSCs are mediated, in part, by their production of IL1 receptor antagonist that limits macrophage inflammation. Now we present data that in addition to soluble mediators, MSC transfer mitochondria and extrude micro RNA (miRNA) ladened exosomes as a mechanism of cell communication to reprogram the innate immunity during lung fibrosis. Our central Hypothesis is that MSCs employ microvesicles as a mean to deliver peptides, miRNAs, and mitochondria to reprogram the innate immunity and ameliorate silicosis. We propose the following specific Aims: 1) To determine whether MSC-derived microvesicles account for the effects of MSC on silica stimulated macrophages. MSC use micro vesicle production to establish a cell independent mechanism of communication with macrophages to reprogram their immune activity. We will determine whether administration of MSC-derived microvesicles into silica-exposed mice is as effective as intact MSC cells in preventing accumulation of Ly6C/Ghi macrophages in the lung thus protecting mice from silicosis. 2) To determine the role of miRNA transfer in MSC-mediated reprogramming of innate immunity during silicosis. MSC-derived microvesicles contain miRNAs and these are highly conserved among different human MSCs. We will determine whether specific miRNA transcripts inside MSC-derived microvesicles recapitulate the micro vesicle-mediated reprogramming of macrophages and determine if inhibiting miRNA production in MSCs eliminates MSC or micro vesicle-mediated responses in macrophages. Gain and loss-of-function studies will be done to define the role of select miRNAs in regulating macrophage function. 3) To define the role of autophagy and coat protein complex II (COPII) as determinants of MSC mitochondrial transfer. MSCs transfer their mitochondria to surrounding macrophages and that mitochondria like structures are present in MSC-derived microvesicles. We will examine whether autophagy and COPII coat systems regulate the load of mitochondria into microvesicles. We will test the hypothesis that mitochondrial transfer could be recapitulated by incubation of macrophages with MSC-derived microvesicles and restore the enzymatic activity of mitochondrial complex I in silica-exposed macrophages. Finally, we will determine whether transfer of mitochondria occurs in the lung of silica-exposed mice.